In many applications, it is necessary to discern the transformation of an input image from an original state. This is the case, for example, in decoding certain digital watermarks.
If the original image is available, certain image features in the original image can be matched with corresponding features in the input image so as to discern any intervening transformations. Such a system is shown in the present assignee's U.S. Pat. No. 5,930,377.
If the original image is not available, other techniques must be employed. In U.S. Pat. No. 5,862,260, and in copending application Ser. No. 09/452,023, filed Nov. 30, 1999 (now U.S. Pat. No. 6,408,082), the present assignee detailed a system employing a subliminal digital graticule signal, having known frequency domain characteristics, that is embedded in the original image with the watermark. On analyzing an input image, the frequency domain characteristics of the graticule signal can be detected and, through application of the Fourier-Mellin transform and generalized matching function (GMF) techniques, the offset, scale and rotation of the image can be discerned. Similar approaches were thereafter detailed by O'Ruanaidh et al in “Rotation, Scale and translation Invariant Digital Image Watermarking,” Int. Conf. on Image Proc., October 1997, IEEE, pp. 536-539; “A Secure Robust Digital Image Watermark,” Proc. SPIE, Vol. 3409, pp. 150-63, 1998; and laid-open application EP 905,967; and by Xerox in U.S. Pat. No. 5,949,055.
While the approach detailed in the '260 patent and its followers is satisfactory for determining offset, scale, and rotation of an input image from a reference state, it cannot discern differential scaling. Differential scaling occurs when an image is “stretched” (or shrunk) differently in the x- and y-directions, and can significantly impair (or prevent) detection of watermark data.
Differential scaling can arise in various ways. One is through deliberate image manipulation. Another is through distortion of a medium on which an image is printed (e.g., paper currency may be stretched through its handling by ATMs and other automated handling equipment). Differential scaling can also arise through scanning. Scanners typically scan documents by sampling successive rows of an image with a linear array of sensors. If the spacing between successive rows does not precisely match the spacing between adjoining sensors, the pixels are not truly square, again resulting in differential scaling.
The below-detailed technology addresses certain of the above-noted shortcomings. For example, some embodiments permit differential scaling to be discerned from an input image, together with offset, scale, and rotation. Other embodiments permit image rotation to be determined without reference to any embedded calibration signal.
The foregoing and additional features and advantages will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.